The present invention relates to polybenzazole that can be formed into a fiber or a film having high strength and high heat resistance, and to a fiber obtained therefrom.
Polybenzazole represented by polybenzobisoxazole, polybenzobisthiazole and polybenzobisimidazole can be formed into a fiber or a film having high strength (e.g., high tensile strength), high elastic modulus (e.g., high tensile modulus) and high heat resistance. For the high strength and high elastic modulus of polybenzazole to be expressed, it is preferable that the reaction for the production of polybenzazole be conducted to afford a number average polymerization degree of not less than 100, and the fiber or film be formed from a dope having a high polymer content affording liquid crystallinity. For this end, polybenzazole needs to be produced in polyphosphoric acid made to have a high concentration by the use of diphosphorus pentaoxide, as described in WO84/01160. The polymer dope resulting from this method contains a polymer having a rigid molecular chain at a high concentration. As a result, its viscosity becomes extremely high and processing thereof into a fiber becomes very difficult. Utilizing the superior physical properties, the spinning rate may be set high and the filament denier of a fiber may be made small, with the consequence that thread breakage occurs frequently and the productivity becomes significantly low.
To solve the problem of low productivity, improvements in connection with the apparatus have been made. For example, JP-A-7-157918 proposes the use of a nozzle having a revised shape for the production of a fiber, thereby to increase productivity by producing a fiber having a small filament denier at a high speed. However, the proposed improvement in the aspect of apparatus is insufficient to provide a desired level of the effect, suggesting the need to improve polybenzazole itself.
It is therefore an object of the present invention to provide polybenzazole suitable for stable production of a fiber having a small filament denier at a high speed without suffering from thread breakage, and a fiber made therefrom.
According to the present invention, there is provided polybenzazole capable of accomplishing the above-mentioned object, which contains not more than 0.010 wt % of an unreacted BB-PBZ monomer.
The present invention provides polybenzazole obtained by dehydration polymerization condensation of an AA-PBZ monomer of the following formula (I) and a BB-PBZ monomer of the following formula (II), which polybenzazole has a residual content of the BB-PBZ monomer of not more than 0.010 wt %.
AA-PBZ monomer 
wherein Ar is a quadrivalent aromatic organic residue and X is O, S or NH;
BB-PBZ monomer
Wxe2x80x94Zxe2x80x94Wxe2x80x83xe2x80x83(II)
wherein Z is an optionally substituted divalent aromatic organic residue and W is a carboxyl group or a group derived from the carboxyl group, which is reactive with -XH in the AA-PBZ monomer.
The preferable embodiments are as follows. The above-mentioned polybenzazole wherein the AA-PBZ monomer is 4,6-diaminoresorcinol. The above-mentioned polybenzazole wherein the BB-PBZ monomer is terephthalic acid. The above-mentioned polybenzazole which is polyparaphenylene-benzobisoxazole. The above-mentioned polybenzazole which has a residual BB-PBZ monomer content of not more than 0.005 wt %. The above-mentioned polybenzazole which has a number average polymerization degree of not less than 30. The above-mentioned polybenzazole which has a number average polymerization degree of not less than 200.
The present invention also provides a fiber obtained by forming the above-mentioned polybenzazole.
In a preferable embodiment, the fiber has a tensile strength of not less than 35 g/d.